The present invention relates to methods and apparatus for non-surgical alteration of corneal curvature in the human eye.
Refractive errors in the function of the eye are quite common in the human population. In fact, moderate levels of far-sightedness ("hyperopia"), near-sightedness ("myopia"), and astigmatism are so widespread as to be considered normal. Less common are the pathological cases which can be severe or degenerative in nature. While Telatively mild refractive errors often can be corrected by external lenses, larger and more complex refractive errors are more difficult and sometimes impossible to correct with external refraction alone.
In the field of surgery, a known technique for treatment of certain forms of refractive errors, such as acute myopia , hyperopia, and astigmatism is to surgically remove an anterior segment of the cornea down into the stroma, to reshape the removed segment as by surgical grinding in a frozen state, and to restore the reshaped segment into the eye. In this type of operation, known as keratoplasty, the eye heals by reformation of the outer epithelium layer over the reshaped stroma.
Alternatively, a layer of the cornea can be opened as a flap, an artificial or donor lenticular implant then inserted under the flap, and the flap sutured up again.
Such invasive corneal procedures are typically limited to treatment of severe conditions, and are generally viewed as a procedures of last resort because of the attendant surgical risks. Nonetheless, a substantial portion of the eye's refractive power is determined by the corneal curvature and reshaping the cornea has been the object of much research and experimental efforts as a means for correction of refractive errors.
Thermokeratoplasty, a class of procedures involving the application of heat to the cornea, has been proposed for hyperopic correction of optical defects. The collagen which forms the corneal stroma is known to shrink by about one third of its initial length when treated to a temperature between about 60.degree.-70.degree. C. This shrinkage appears to be permanent, with the potential of little or no lasting opacity of the treated site resulting from the treatment. Hence, various thermokeratoplasty techniques seek to exploit such collagen shrinkage profile, including inserting a nicrome wire into the cornea and heating the surrounding collagen tissue to a non-damaging temperature to cause permanent shrinkage of the collagen tissue. This shrinkage beneficially changes the curvature of the cornea. Other techniques include application of RF current or laser energy to effect permanent change in the corneal collagen.
A problem with heating of the eye lies in the possibility of damage to the epithelium and Bowman's membrane on the anterior side of the cornea, as well as Descemet's membrane and the endothelium on the corneal posterior, as heat energy is applied to develop a critical shrinkage temperature in the internal stromal collagen. It is therefore desirable to minimize the heating effect in these sensitive membranes, and particularly in the endothelium, while still obtaining the desired 60.degree.-70.degree. C. temperature range in the stroma. Thus, the treatment duration and other paremeters that affect the thermal dose to the cornea must be precisely controlled in any treatment delivery system.
It is therefore an object of the present invention to provide a non-surgical and non-damaging thermal treatment method and apparatus for the correction of the refractive power of the cornea.
It is another object of the present invention to provide a method and apparatus particularly useful for hyperopic and astigmatic correction by selectively applying heat to the cornea to induce volumetric coagulation in the corneal collagen and thereby steepen regions of the central cornea.